1. Field of the Invention
This invention relates to the field of servo patterns for information storage media.
2. Background Art
Information is written on data storage disks in concentric tracks which are accessed by read/write heads. To accurately access the data tracks, servo information is also written onto storage disks and provides positioning information. It too is ordinarily written in concentric tricks. In a multi-disk storage environment, one entire side of a storage disk may be dedicated to servo information. A servo head accesses this servo disk to read the position information therein. Since the servo head is in a fixed relationship relative to the read/write heads, the position of the servo head can be used to indicate the position of the read/write heads. In addition to having a dedicated surface for servo information, a "sector" servo pattern may be employed in which pie shaped wedges of servo information are interleaved between sections of data information.
These are three basic types of servo patterns currently employed in magnetic disk storage schemes, namely amplitude based, frequency based and pulse detection based. Amplitude based servo patterns rely on differences in amplitude between successive sections of the servo pattern to provide position information. Frequency based servo patterns typically use side by side servo tracks of different frequencies. The interface between these frequencies is a nominal track center. A servo head compares the amplitude of the two different frequencies to determine the track center. Pulse detection based servo patterns utilize dibit, tribit, or quadbit patterns to provide position information.
In order to improve the efficiency of data storage in magnetic disk systems, it is desired to maximize the data storage capability. Any disk surface area which is dedicated to servo tracks cannot be used for data tracks. One method of freeing up space for data tracks is to "bury" the servo layer beneath the surface of the data disk itself. There are several disadvantages associate with the use of prior art servo patterns in a buried servo scheme. One disadvantage is a large swing in amplitude due to disk media variations, amplitude reduction as track density is increased and amplitude detection problems inherent with the use of a buried servo layer. Further, adequate frequency separation between the data tracks and the servo tracks must be provided to allow the decoding circuitry to separate the signals and thereby prevent cross talk. However, such frequency separation may result in a servo pattern lacking the desire tracking accuracy.
Therefore, it is an object of the present invention to provide a servo pattern for use with a buried servo scheme which is amplitude and pitch independent.
It is a further object of the present invention to provide a servo pattern which provides adequate separation of data and servo signals to prevent cross talk.
It is yet another object of the present invention to provide a servo pattern providing highly accurate tracking and position information.